In a conventional power transmission mechanism for an engine, through an engine accessory belt trained around an idler pulley, power is transmitted from the crankshaft of the engine to an engine accessory such as a water pump (WP) or an idling stop generator (ISG). In this case, the engine accessory always rotates while the crankshaft is rotating. This means that the water pump is rotated even when it does not need to be rotated such as while the engine is being rotated for warm-up. This leads to the wear of the accessory belt, and to the increase of fuel consumption resulting, e.g., from the unnecessary rotation of the pulley.
In order to overcome this problem, as illustrated in FIG. 1 of Japanese Patent No. 4891914, it is known to use, instead of the accessory belt, an idler roller (friction wheel 17) arranged between a driving roller (crankshaft pulley 4) and a driven roller (friction pulley 14) so as to transmit the rotational force of the driving roller to the driven roller due to the frictional forces between the idler roller and the respective driving and driven rollers. Unlike the engine accessory belt, the idler roller can selectively come into contact with and separate from the driving roller and the driven roller by moving back and forth.
A mechanism (cam actuator) configured to move the idler roller back and forth so as to uniformly bring the idler roller into abutment with the driving roller and the driven roller is now described with reference to FIG. 1 of Japanese Patent No. 4809341. The cam actuator illustrated in FIG. 1 of JP 4809341 includes a planetary speed reducer R for reducing the rotation of a motor 1, and an eccentric cam 3 for converting the reduced rotation of the motor 1 into the reciprocating motion of a coupling rod 201, thereby moving, back and forth, a pulley 300 supported at an end of the coupling rod 201. By moving the pulley 300 back and forth according to the operating conditions of the engine, it is possible to control the transmission and stop of power from the driving roller to the driven roller, and thus to improve fuel economy.
The coupling rod 201 is configured to be slightly pivotable from the axial direction at its intermediate portion. Since the coupling rod 201 is pivotable in this way, when the pulley 300 abuts against the driving roller and the driven roller, the coupling rod 201 pivots such that the abutment force between the pulley 300 and the driving roller is substantially equal to the abutment force between the pulley 300 and the driven roller, thereby positioning the pulley 300 at the optimum position.
The cam actuator according to Japanese Patent No. 4809341 is configured to grip the rotary shaft of the pulley 300 (idler roller) and pivot the rotary shaft about a point located outside of the pulley 300 (substantially at the longitudinal center of the coupling road 201). Therefore, an extra space is necessary around the pulley 300 in which the pulley 300 can pivot. Such an extra space makes it difficult to make the power transmission mechanism small-sized, and to freely lay out the power transmission mechanism. Since the pulley 300 is positioned by pivoting the single coupling rod 201, the coupling rod 201 might be twisted about its axis. This tends to cause the pulley 300 to be brought into abutment with the driving roller and the driven roller in an undesirable manner such that power cannot be transmitted stably due to friction.
Moreover, the driving and driven rollers may not have a complete circular shape. If the driving and driven rollers do not have a complete circular shape, resonance or self-excited vibration is likely to occur as the driving and driven rollers rotate. Therefore, it is impossible to stably keep the pulley 300 in contact with the driving and driven rollers, and thus to stably transmit power from the driving roller to the driven roller.
It is an object of the present invention to provide a power transmission roller assembly capably of stably transmitting power between a driving roller and a driven roller of a power transmission mechanism due to the friction therebetween, and to make the power transmission mechanism small-sized.